1. Field of the Invention
The present invention relates to a method of projecting printing fluid droplets towards a printing surface, and particularly, to such a method in which projection is controlled by regulating the viscosity of the printing fluid, and further relates to a modified ink jet recorder constructed so as to operate in accordance with the disclosed method.
2. Description of the Prior Art
Ink jet systems, and particularly impulse ink jet systems, are well known in the art. Basically, these impulse systems utilize short pressure pulses to eject ink droplets from an ink chamber through a small orifice or nozzle onto a surface in a specific pattern to form an image. Each droplet results from a pressure wave in the fluid, produced by applying a voltage pulse to a transducer composed, by way of example, of a piezo-electric ceramic material. The term "impulse" or "drop-on-demand" as used in the prior art and in this application refers to ink jet systems in which there is no restriction on the rate (frequency) of ink droplet ejection, other than the recovery time needed to refill the nozzle. That is to say, droplets may be ejected at any desired rate, with or without a pattern, sequence or rhythm.
The principle of an impulse ink jet is the compression of ink and the subsequent emission of ink droplets from an ink chamber through a nozzle or orifice by means of a pump or driver mechanism which is composed of a transducer material (for example, a piezo-ceramic) bonded to a thin diaphragm. When a voltage is applied to the piezo-ceramic material, the material attempts to change its planar dimensions, but because it is securely and rigidly attached to the diaphragm, bending occurs. In an impulse jet, the change in dimensions of the transducer-diaphragm structure due to an electrical impulse is used to apply pressure to the ink. A typical drive voltage required for a 100 micrometer thick transducer to force ink droplets through a nozzle in an impulse fashion might be 100 volts. The impulse might last 20-40 microseconds and produce a driver displacement of 100 micrometers with a resulting pressure of one atmosphere. Refill of the ink after a droplet emerges from the nozzle results from the capillary action at the nozzle. Refill of the jet customarily requires about 100 microseconds, but depends upon the viscosity and surface tension of the ink as well as the impedance of the fluid channels. A negative hydrostatic pressure of about one inch balances the capillary attraction.
Typical disclosures of known impulse ink jet methods and apparatus are presented in the several U.S. Pat. Nos. to Kyser et al, Nos. 3,946,398, 4,189,734, 4,216,483 and 4,339,763. According to those disclosures, fluid droplets are projected from a plurality of orifices or nozzles at both a rate and in a volume controlled by electrical signals. In each instance, each nozzle or orifice requires an associated pump or driver mechanism.
In another known instance, an ink jet system is commercially produced by Hewlett-Packard Corporation under the trademark "Bubble Jet" and is disclosed in U.S. Pat. No. 4,490,728 to Vaught et al. According to the Bubble Jet concept, a heater located behind and spaced from the nozzle raises the temperature of the printing fluid to above the boiling point. The printing fluid thereby changes state from liquid to gas. This causes a bubble to form which displaces the printing fluid and creates a pressure pulse which, in turn, forces a droplet out of the nozzle. Subsequently, the bubble collapses, causing cavitation and, in time, heater degradation. With continued use, the ink jet must eventually be replaced. Another disclosure of this nature is found in earlier U.S. Pat. No. 4,337,467 to Yano.
Exxon Corporation, also, produces a commercial ink jet printer under the trademark Exxon 965 Ink Jet Printer which operates with an oil base ink having a viscosity of approximately 60 cp at room temperature. In that instance, the entire jet head is heated, and not merely individual droplets or nozzles. The higher viscosity ink is reportedly used because it is easier to handle, and specifically, because it does not develop bubbles when it is jostled during transport.
Numerous other patents disclose thermal ink jet printers. Among these are U.S. Pat. No. 4,450,457 to Miyachi et al, No. 4,251,824 to Hara et al which discloses change of state of the liquid to develop a foam, and No. 4,490,731 to Vaught which discloses change of state of the ink dye vehicle from the solid to the liquid state.
In conventional practice, an array of ink jets or ink jet heads requires an associated array of transducers, one transducer for each ink jet. Typically, each transducer is separately mounted adjacent the ink chamber of each jet by an adhesive bonding technique. This presents a problem when the number of transducers in the array is greater than, for example, a dozen because complications generally arise due to increased handling complexities, for example, breakage. In addition, the time and parts expense rise almost linearly with the number of separate transducers that must be bonded to the diaphragm. Furthermore, the chances of a failure or a wider spread in performance variables such as droplet volume and speed, generally increase.